If you are using KDE, and find Recently Used section in your menu, you probably feel it is annoying. Especially for your privacy. So how to disable it? There are two methods, temporarily and permanently. I use KDE 4.10.3 (Ubuntu 12.04) here and you may use this article until KDE 5.

Temporarily

Permanently

Do this command:

chmod u-w ~/.kde/share/apps/RecentDocuments/

This command means user minus write (forbid user to write) for KDE RecentDocuments directory. If you forbid any writing attemp into RecentDocuments, automatically it will be disabled in KDE Menu. This is applicable in KDE for Ubuntu. If you are using another distro (e.g. openSUSE), maybe you must use .kde4 instead of .kde directory.